Method and device for producing a microalloyed steel, in particular a pipe steel

ABSTRACT

The invention relates to a method of making microalloyed steel, in particular a pipe steel, wherein a cast slab ( 1 ) passes through an installation ( 2 ) having a casting machine ( 3 ), a first furnace ( 4 ), at least one roughing roll stand ( 5 ), a second furnace ( 6 ), at least one finishing roll stand ( 7 ), and a cooling line ( 8 ) in this order in the travel direction (F) of the slab ( 1 ). The method comprises: a) definition of a desired temperature profile for the slab ( 1 ) over its travel through the installation ( 2 ); positioning in the process line (L) of the installation ( 2 ) at least one temperature-influencing element ( 9, 10 ) for setting the temperature of the slab ( 1 ) according to the defined temperature profile, the temperature-influencing element ( 9, 10 ) being introduced between the first furnace ( 4 ) and the roughing roll stand ( 5 ), and/or between the second furnace ( 6 ) and the finishing roll stand ( 7 ); production of the slab ( 1 ) in the installation ( 2 ) configured in this manner, the temperature-influencing element ( 9, 10 ) being operated in such a way that the defined temperature profile is at least substantially maintained. The invention further relates to an installation for making microalloyed steel.

The invention relates to a method of making microalloyed steel, in particular pipe steel, where a cast slab passes through an installation having a casting machine, a first furnace, at least one shaping roll stand, a second furnace, at least one finishing roll stand, and a cooling line in this order in the travel direction of the slab. The invention further relates to an installation for making microalloyed steel.

Various options are described in the prior art for manufacturing a strip according to the generic method. Reference is made to US 2005/0115649 A1, WO 2009/012963 A1, WO 2007/073841 A1, WO 2009/027045 A1, EP 0 611 610 B1, and EP 1 860 204 A1, for example.

Thermomechanical rolling is an established method. Microalloyed steels have become increasingly important in recent times. Pipe steel (according to API Specification 5L) is one of the most important subgroups within microalloyed steel. The demand for such steel is constantly increasing.

Most pipe steel is produced in plate rolling mills. However, in particular if the final thicknesses and final widths are not too great, pipe steel may also be produced in hot-roll wide strip mills, referred to as CSP facilities, and other apparatuses for hot rolling.

To make microalloyed steel in general and pipe steel in particular, special emphasis is placed on the variation of temperature as a function of time (or a function of the location within the production installation). This variation, in combination with the reduction distribution, has a crucial influence on development of the microstructure, and therefore specific mechanical and technical properties of the steel. For this reason high-powered cooling units, for example, are used downstream of the finishing stage, thus allowing setting of the desired temperature variation.

It is disadvantageous that previously known production apparatuses and methods are not optimally suited for flexibly responding to the particular starting conditions and requirements in the manufacture of microalloyed steel, in particular pipe steel, in order to produce these types of steel with an essentially freely selectable temperature profile over time or over the production path. Therefore, it is not optimally possible to control and influence structural development within the steel. The flexible manufacture of this steel with regard to its chemical composition and dimensions is therefore limited.

The object of the present invention, therefore, is to provide a method and an associated apparatus that allow the stated disadvantages to be overcome. Accordingly, it is the aim to enable improved control of the variation of temperature according to a desired profile over time or over the production path in order to allow better monitoring and control of the structural development. Thus, a further goal is to enable more flexible production of microalloyed steel, in particular pipe steel.

With regard to the method, this object is achieved by the invention as characterized by the following sequential steps:

a) definition of a desired temperature profile for the slab over its travel through the installation;

b) positioning in the process line of the installation at least one temperature-influencing element for setting the temperature of the slab according to the defined temperature profile, the temperature-influencing element being introduced between the first furnace and the roughing roll stand, and/or between the second furnace and the finishing roll stand;

c) production of the slab or strip in the installation configured in this manner, the temperature-influencing element being operated in such a way that the defined temperature profile is at least substantially maintained.

According to one embodiment of the invention, an additional furnace is used as the temperature-influencing element. This may be an induction furnace or a furnace that heats the slab by direct flame action (DFI oxyfuel furnace). In the latter case, the direct flame action of the slab is provided by a gas jet containing 75% oxygen mixed with a gaseous or liquid fuel. A soaking furnace, a roller hearth furnace, or a walking-beam furnace, i.e. pusher-type furnace, may also be used as the additional furnace.

An additional cooling line may also be used as the temperature-influencing element. This may be an intensive cooler or a laminar strip cooler, for example.

Finally, a thermally insulating element (roller grippers) may also be used as the temperature-influencing element.

The temperature profile is preferably determined on the basis of a structural model. The structural model preferably specifies and/or monitors the following parameters: the temperature profile over time or the number of passes, the reduction distribution over time or the number of passes, the holding or cycle times, the roller speeds and transport speeds, and/or the heating and cooling intensities.

One refinement provides that by use of a temperature-influencing element in the form of a cooler, an input temperature may be achieved in the finishing roll stand that is low enough that the recrystallization and grain growth at that location are largely halted, the temperature level between the input into the roughing roll stand and the input into the temperature-influencing element in the form of a cooler being

a) decreased, in particular for pipe steel having a low microalloy element content and small slab thickness, by a temperature-influencing element in the form of a cooler in order to reduce the grain size upon entry into the finishing roll stand, or

b) increased, in particular for pipe steel having a high microalloy element content and large slab thickness, by a temperature-influencing element in the form of a heater to ensure complete recrystallization during roughing, or

c) merely balanced and otherwise left unchanged.

According to one refinement, using a temperature-influencing element in the form of a heater makes it possible to achieve an input temperature in the finishing roll stand that is high enough that complete recrystallization takes place at that location, either

a) during the first finishing passes due to the high temperatures and reductions, followed by an accumulation of deformation in the last finishing passes, or

b) only during the last finishing passes due to moderate temperatures and reductions, after prior accumulation of deformation has taken place.

According to the invention, the installation for making microalloyed steel, in particular a pipe steel, having a casting machine, a first furnace, at least one roughing roll stand, a second furnace, at least one finishing roll stand, and a cooling line in this order in the travel direction of the slab, is is characterized in that a temperature-influencing element for setting the temperature of the slab may optionally be introduced into the process line between the first furnace and the roughing roll stand, and/or between the second furnace and the finishing roll stand, the temperature-influencing element being selectable from one of the following elements: an additional furnace, an additional cooling line, or a thermally insulating element.

One refinement provides that at least one of the temperature-influencing elements comprising the additional furnace, additional cooling line, and thermally insulating element is made a transversely displaceable with respect to the travel direction of the slab in such a way that one of the elements may optionally be introduced into the process line.

At least one of the elements comprising the additional furnace, additional cooling line, and thermally insulating element may be configured to be pivotal about an axis extending in the travel direction in such a way that one of the elements may optionally be introduced into the process line.

By using the proposed approach, improved manufacture of microalloyed steel, in particular pipe steel (such as X52 to X120, for example), is possible, resulting in favorable property combinations. Optimal values of strength and ductility, in addition to maximum flexibility with regard to the chemical compositions used, as well as the dimensions of the final product are achieved as the result of targeted control of the temperature variation. The limitations that have existed heretofore on the basis of customary process control may be largely eliminated using the proposal according to the invention. The application of a desired temperature-time curve is advantageously achieved for manufacture of the steel, thus allowing production of extremely high-quality pipe steel.

According to the proposed procedure, upstream of the roughing stage as well as between the roughing stage and the finishing stage the temperature may be increased, held constant, or decreased. Maximum flexibility with regard to temperature control is thus achieved that opens up not only the basic possibility for manufacturing pipe steel, but also allows different procedures for manufacturing these types of steel and setting various material properties, depending on requirements.

In addition, many other types of steel for which the temperature variation plays an important role may be manufactured much more easily, and in certain cases, with improved properties, which is the case for multiphase steels and all types of microalloyed steel, for example.

Finally, changed reduction distributions may be used, and in particular higher reductions may be carried out by the altered temperature variations. This also results in lower achievable final thicknesses for all types of steel, and also additional possibilities for installation design.

The use of effective heaters (inductive heaters or furnaces according to the DFI oxyfuel process) and/or the use of adjustable intensive coolers (for example, instead of exposing roughed strips in air) further increase the overall productivity of the installation, or simplify the production process.

Thus, the proposed procedure or apparatus allows targeted influencing of the temperature of the slab before roughing, on the basis of material analysis, material dimensions, and material properties. Likewise, targeted influencing of the temperature of the roughed strip before finish milling on the basis of material analysis, material dimensions, and material properties is possible.

Temperature control during the individual process steps is preferably done in a targeted manner by use or application of a structural model. As previously mentioned, the structural model sets the course of and monitors the following parameters:

-   -   Temperature profile over time or a number of passes,     -   Reduction distribution over time or a number of passes,     -   Holding or cycle times,     -   Roller speeds and transport speeds for influencing the         temperature profile,     -   Heating and cooling intensities.

Furthermore, targeted control of the various types of softening processes during the individual process steps and associated control of the material properties may be achieved.

The method may be used for various thermomechanical treatments.

Slab cooling may be incorporated upstream of the roughing of the slab in the roughing roll stand. Likewise, an induction heater or a DFI oxyfuel heater may be incorporated upstream of the roughing in the roughing roll stand.

The various cooling and heating units may be exchanged for one another by sliding or pivoting out.

The maximum achievable reductions and the overall reduction distribution may be influenced by a targeted temperature increase upstream of the rough milling and finish milling, with effects on the dimensions and properties of the product and the installation design.

The productivity of the rolling installation is thus increased by targeted (additional) cooling and/or heating.

Embodiments of the invention are illustrated in the drawings, in which:

FIG. 1 schematically shows a casting rolling installation in side view according to a first embodiment of the invention, having a casting machine, first furnace, roughing stage, second furnace, finishing stage, and cooling line(s);

FIG. 2 shows a design alternative to FIG. 1 of the casting rolling installation according to a second embodiment;

FIG. 3 shows a further design alternative to FIG. 1 of the casting rolling installation according to a third embodiment;

FIG. 4 shows a further design alternative to FIG. 1 of the casting rolling installation according to a fourth embodiment;

FIG. 5 shows a further design alternative to FIG. 1 of the casting rolling installation according to a fifth embodiment;

FIG. 6 shows a further design alternative to FIG. 1 of the casting rolling installation according to a sixth embodiment;

FIG. 7 schematically shows a casting/rolling installation in top view according to a further embodiment;

FIG. 8 schematically shows temperature-influencing elements of the casting rolling installation, viewed in the travel direction of the slab, according to a first embodiment of the invention;

FIG. 9 shows a further design alternative to FIG. 8 of the temperature-influencing elements according to a second embodiment of the invention;

FIG. 10 shows a further design alternative to FIG. 8 of the temperature-influencing elements according to a third embodiment of the invention; and

FIG. 11 shows a further design alternative to FIG. 8 of the temperature-influencing elements according to a fourth embodiment of the invention.

FIG. 1 shows an installation 2 for casting and rolling in a line of pipe steel (according to API Specification 5L), in the side view. The installation has a casting machine 3 (vertical casting installation or curved casting installation) in which a slab 1 is produced in a known manner by continuous casting. A thickness between 50 and 150 mm and a width between 900 and 3000 mm are typical dimensions of the slab. In a travel direction F, the casting machine 3 is followed by a first furnace 4, a roughing stage for rolling the slab, only a single roughing roll stand 5 being illustrated (multiple roughing roll stands are sometimes provided), a second furnace 6, a finishing stage for rolling the slab or strip, only a single finishing roll stand 7 being illustrated (multiple finishing roll stands are usually provided), and a cooling line 8.

Further elements are also present that are not important or that are only secondary with regard to temperature control. Shears 12 are provided between the casting machine 3 and the first furnace 4, by means of which the slab 1 may be cut to a desired slab length (alternatively, a flame cutting installation may be used). A descaling sprayer 13 is provided between the first furnace 4 and the roughing roll stand 5. An additional descaling sprayer 14 is located directly upstream of the finishing roll stand 7. A reel 15 that winds the finished strip is provided in a known manner downstream of the cooling line 8.

For pipe steel there are increased demands with regard to temperature control of the slab or strip on the path through the installation 2.

Before the strip is manufactured, first the desired temperature profile over time or along the production path in the travel direction F is determined. For this purpose a computerized structural model is preferably used that is known per se and is used by those skilled in the art to specify the intended temperature curve of the slab 1 or strip so that an optimal product may be produced. Examples of such temperature variation are given below, in that temperature ranges of the slab 1 or strip are provided for particular locations in the production installation 2.

Depending on the specified temperature profile, the installation 2 is to be prepared in such a way that the desired profile may be followed. According to the invention, this is achieved in such a way that at least one temperature-influencing element for setting the temperature of the slab 1 according to the defined temperature profile is provided in the process line of the installation 2, the temperature-influencing element being introduced between the first furnace 4 and the roughing roll stand 5, and/or between the second furnace 6 and the finishing roll stand 7.

In the embodiment according to FIG. 1, the temperature-influencing element 9 is a cooling line that is effectively introduced into the process line, downstream of the second furnace 6. This may be an intensive cooler or a laminar cooler, depending on the cooling power required for achieving the desired temperature profile.

After cooling and passing through the descaling sprayer 14, continuous or reversible finish milling is carried out in the finishing roll stand 7, a number of finishing roll stands, i.e. a finishing roll stand group, preferably being provided. The finish milling is carried out to the desired finished strip thickness and finished strip temperature, followed by cooling of the strip in the cooling line 8. As the last step, the strip is wound onto the reel 15. Alternatively, instead of winding the finished rolled strip, the strip may be sent directly to the finishing shop.

A temperature range from 850 to 950° C. downstream of the furnace 6 and the cooler 9 is provided for finish milling pipe steel within the scope of a classical thermomechanical treatment. The low input temperature ensures that during the essentially isothermal rolling in the finishing stage, recrystallization and grain growth are largely halted, and practically the entire deformation is accumulated, resulting in a very finely grained is structure in the subsequent conversion. Other prerequisites are a sufficiently low final rolling temperature of typically less than 820° C. and a sufficiently high cooling rate in the cooling line.

However, in addition to the above-described cooling in the region between the roughing roll stand 5 and the finishing roll stand 7, it may be necessary to influence the temperature of the strip before entry into the roughing roll stand 5. In this regard, FIG. 2 shows an installation 2 for manufacturing pipe steel according to API in which the downstream section of the first furnace 4 has been replaced by a strip cooler 10. Stated more precisely, in this case an additional cooling line 10 has been introduced into the process line as the temperature-influencing element 10.

Cooling the slab further increases the extent of the thermomechanical treatment and may limit grain growth between the shaping mill and finish roll stands. However, complete recrystallization must still be ensured, which makes this procedure suitable in particular for pipe steel having a low content of microalloy elements and fairly small slab thicknesses.

In contrast, for particularly high content of alloy elements and large slab thicknesses, heating to even higher temperatures may be useful to allow higher deformation rates and to ensure complete dynamic or static recrystallization. In addition, the increased temperature may have a favorable effect on the solution condition of the microalloy elements. One embodiment of the invention that makes this possible in a particularly advantageous manner is illustrated in FIG. 3. Here, a temperature-influencing element 10 in the form of an inductive heater has been introduced into the process line, downstream of the first furnace 4 and upstream of the roughing roll stand 5.

FIGS. 4, 5, and 6 show installation designs in which the main difference from the approach according to FIGS. 2 and 3 is that the strip cooler provided upstream of the finish milling has been replaced by an induction heater or furnace.

Whereas a classical thermomechanical treatment having the aim of maximizing the accumulated deformation has been sought heretofore, for certain steel a different method is to be used. Instead of dispensing with further softening in the region of the finishing stage after complete recrystallization following roughing, repeated recrystallization is the aim. This recrystallization requires high temperatures that may be produced in a particularly advantageous manner using an induction heater or a DFI oxyfuel furnace. The recrystallization may be carried out at particularly high temperatures and deformation rates during the first finishing passes, and may be followed by accumulation of deformation in the last finishing passes, or alternatively, dynamic recrystallization occurs at lower temperatures and deformation rates only during the last finishing passes, after accumulation of deformation in the first finishing passes has taken place. In comparison to the classical thermomechanical treatment, for example according to the approaches according to FIGS. 1, 2, and 3, in both of the above-mentioned cases as a result of the temperature increase the maximal possible deformation rate increases while the deformation required for initiating recrystallization decreases, is thus greatly increasing the tendency toward softening.

As the result of expanding the options according to the invention for influencing temperature, previous conflicting requirements for the temperature variation may be met in the individual installation zones, thus enabling the optimal course of the process in each individual zone with regard to the product properties, i.e. following an optimally selected temperature curve in the slab or strip along the travel direction F. This provides a flexible adaptation to desired material properties, material dimensions, or different material analyses.

At the same time, influencing temperature control is a powerful tool for distributing the load and reduction in the shaping and finishing roll stands used to reduce the minimum achievable final thicknesses, or to employ smaller units in the design.

The description of the numerous effects of the temperature variation on the microstructure illustrates that control of the structural development is necessary at all times, and that the rolling of pipe steel according to the proposed procedure results in the desired mechanical properties in particular when the process is monitored and/or controlled or regulated by a suitable structural model.

For rolling standard steel in the same installation, temperatures of approximately 1000 to 1150° C., or in special cases higher or lower temperatures, are generally used upstream of the finishing stage. The necessity of setting differing temperatures becomes greater with increasing complexity of the alloy design. This procedure is particularly advantageous for multiphase steels and various microalloyed steel. By using the proposed installation design, in most cases slabs, thin slabs, intermediate strips, strips, and sheets may be brought to the sought temperature level, so that there are no restrictions with regard to the required material properties.

For optimal adaptation to the particular process conditions, the strip cooler 9 (in FIGS. 1, 2, and 3) and the induction heater 10 (in FIGS. 3 and 5) or 9 (in FIG. 4) are designed to be shiftable or pivotal transverse to the travel direction F, and either one or the other of units 9, 10 may be activated.

Similarly, as an alternative to FIG. 4, according to FIG. 6 instead of the strip cooler 10 or induction heater 9 a conventional soaking furnace 9, 10 may be introduced into the process line. This applies for the various units upstream and downstream of the shaping mill installation.

The casting machine 3 may be provided in the process line with the roll stand 5, or may be spatially separated from it. For this purpose, reference is made to FIG. 7 that shows a corresponding example in the top view. In this case two upper casting machines 3′ are provided in parallel, downstream of which the slab is cut to a desired length by flame cutting machines 12′. By use of a walking-beam furnace 4′ or pusher-type furnace the slab 1 may be displaced from the two upper process lines L to the lower process line L, in a direction Q transverse to the travel direction is F; the further installation parts for manufacturing the strip are located in the lower process line. The lower process line L likewise has a casting machine 3 downstream of which shears 12 are provided.

The slab 1 is heated to a roughing temperature of approximately 1100 to 1200° C. by furnaces 4, 4′. Roughing is carried out downstream of the descaling sprayer 13 continuously or reversibly on one, or alternatively multiple, roughing roll stands 5 to produce an intermediate thickness.

The furnace input temperature may also be influenced by the selection of the roller speed at the roughing roll stand 5.

A second furnace 6 in the form of a holding furnace is provided downstream of the roughing roll stand 5. The holding furnace 6 provides enough space to completely accommodate a thin slab that has been shaped in the roughing roll stand 5. Brief cycling of the shaped thin slab may also take place in the furnace 6.

Instead of a holding furnace 6, in this case roller grippers or a standard roller table may be provided. Subsequent to the furnace 6 or the roller grippers, a temperature-influencing element 9 in the form of a cooling line is positioned in the process line L, by means of which the slab 1 may be brought to the desired temperature before the finish milling in the finishing roll stand 7. Alternatively, the strip cooler 9 may be provided upstream of the holding furnace or the roller grippers.

Details concerning exchange of the various units by lateral displacement or pivoting in and out of the temperature-influencing elements 9, 10 are shown in FIGS. 8 through 11. In addition, three different units may share a space in the process line by optionally providing suitable displacement devices.

FIG. 8 shows the manner in which an additional furnace (left side of FIG. 8) or an induction furnace (right side of FIG. 8) may alternatively be moved into the process line L by displacement in transverse direction Q. Offset positions 16, 16′ on both sides of the process line L allow the two furnaces to be simultaneously moved from the illustrated position to the right, and vice versa.

The analogous situation is illustrated in FIG. 9 for temperature-influencing elements 9, 10 that may alternatively be introduced in the form of a cooler (left side of FIG. 9) and an induction furnace (right side of FIG. 9) into the process line L. Once again, this applies for FIG. 10 for a roller hearth furnace (left) and a slab cooler (right).

As shown in FIG. 11, a temperature-influencing element 9 in the form of a cooling bar may be pivoted about an axis 11 for engagement or disengagement. However, the induction furnace 10 is once again provided so as to be transversely displaceable in direction Q in order to disengage it by moving it into the deflection position 16′.

List of reference numerals:  1 Slab (strip)  2 Installation  3 Casting machine  3′ Casting machine  4 First furnace  4′ Walking-beam furnace or pusher-type furnace  5 Roughing roll stand  6 Second furnace  7 Finishing roll stand  8 Cooling line  9 Temperature-influencing element 10 Temperature-influencing element 11 Pivot axis 12 Shears 12′ Flame cutting installation 13 Descaling sprayer 14 Descaling sprayer 15 Reel 16 Deflection position 16′ Deflection position F Travel direction Q Direction of transverse displacement L Process line 

1-26. (canceled)
 27. A method of making microalloyed steel, in particular pipe steel, wherein a cast slab passes through an installation having a casting machine, a first furnace, at least one roughing roll stand, a second furnace, at least one finishing roll stand, and a cooling line in this order in a travel direction of the slab, the method comprising: a) definition of a desired temperature profile for the slab over its travel through the installation; b) positioning in the process line of the installation at least one temperature-influencing element for setting the temperature of the slab according to the defined temperature profile, the temperature-influencing element being provided between the first furnace and the roughing roll stand, or between the second furnace and the finishing roll stand; c) production of the slab or strip in the installation configured in this manner, the temperature-influencing element being operated in such a way that the defined temperature profile is at least substantially maintained, wherein by use of a temperature-influencing element in the form of a cooler, an input temperature may be achieved in the finishing roll stand that is low enough that the recrystallization and grain growth at that location are largely halted, the temperature level between the input into the roughing roll stand and the input into the temperature-influencing element in the form of a cooler being either aa) decreased, in particular for pipe steel having a low microalloy element content and small slab thickness, by a temperature-influencing element in the form of a cooler in order to reduce the grain size upon entry into the finishing roll stand, or bb) increased, in particular for pipe steel having a high microalloy element content and large slab thickness, by a temperature-influencing element in the form of a heater to ensure complete recrystallization during roughing, or cc) merely balanced and otherwise being left unchanged, or by use of a temperature-influencing element in the form of a heater, an input temperature is achieved in the finishing roll stand that is high enough that complete recrystallization takes place at that location, either Aa) during the first finishing passes due to the high temperatures and reductions, followed by an accumulation of deformation in the last finishing passes, or Bb) only during the last finishing passes due to moderate temperatures and reductions, after prior accumulation of deformation has taken place.
 28. The method defined in claim 27, wherein an additional furnace is used as the temperature-influencing element.
 29. The method defined in claim 28, wherein an induction furnace is used as the additional furnace.
 30. The method defined in claim 28, wherein in the additional furnace the slab is heated by direct flame action.
 31. The method defined in claim 30, wherein the direct flame action of the slab is provided by a gas jet containing 75% oxygen and a gaseous or liquid fuel.
 32. The method defined in claim 28, wherein a soaking furnace is used as the additional furnace.
 33. The method defined in claim 28, wherein a roller hearth furnace is used as the additional furnace.
 34. The method defined in claim 28, wherein a walking-beam furnace or a pusher-type furnace that allows transverse transport of the slab is used as the additional furnace.
 35. The method defined in claim 27, wherein an additional cooling line is used as the temperature-influencing element.
 36. The method defined in claim 35, wherein an intensive cooler is used as the additional cooling line.
 36. The method defined in claim 35, wherein a laminar strip cooler is used as the additional cooling line.
 37. The method defined in claim 27, wherein a thermally insulating element is used as the temperature-influencing element.
 38. The method defined in claim 27, wherein the temperature profile is determined on the basis of a structural model.
 39. The method defined in claim 38, wherein the structural model specifies or monitors the following parameters: the temperature profile over time or the number of passes, the reduction distribution over time or the number of passes, the holding or cycle times, the roller speeds and transport speeds, or the heating and cooling intensities. 